1. Field of the Invention
The present invention relates to semiconductor processing and, in particular, concerns a method of conditioning and measuring pads used in planarizing surfaces of a wafer using chemical-mechanical polishing.
2. Description of the Related Art
Chemical-mechanical polishing (CMP) is a technique whereby surfaces are planarized by the simultaneous application of both an etching and a polishing process. Semiconductor wafers are often globally planarized using CMP processing. In a typical CMP process, the semiconductor wafer is placed on a carriage, and a pad is positioned over the wafer to contact the upper surface of the semiconductor wafer. The carriage and the pad are further rotated in opposite directions, and a slurry containing an etchant and abrasive particles flows between the upper surface of the semiconductor wafer and the pad. The combination of the mechanical polishing of the pad and the chemical etching action involved in this process serves to remove exposed surfaces of the wafer thereby planarizing the upper surface of the semiconductor wafer.
In one implementation, the CMP process is used for demascene processing in which excess layers of copper compounds are removed from the semiconductor wafer surface, leaving only the necessary copper conduit. After a time, newly removed copper compounds, such as Cu(OH)2 and Cu(OH), clog and contaminate the CMP pad, thereby degrading its planarization effectiveness. More specifically, as the CMP pad is used, contaminants build up within the pores or grooves of the CMP pad, thereby inhibiting an even distribution of slurry. This could cause uneven planarization of the wafer and necessary wafer materials might be removed.
To address these problems, methods have been developed to determine the level of contaminants on the CMP pad so as to indicate when the pad is no longer acceptable for use. One such method involves pouring a solvent, such as a solution of 1% nitric acid (HNO3), onto the pad whereby the solvent draws the contaminants from the pad. Then, a sample of this solvent and contaminant mixture is taken from the pad using a pipette, and this sample is tested for the level of contaminants present in the solution using spectrometer technology, such as ion coupled plasma analysis.
This method has several problems. For example, pouring the solution on the pad in an uncontrolled manner may actually wash away contaminants from the area that will ultimately serve as the collection point. As a result, the sample taken may not be a representative sample of the level of contaminants actually on the rest of the pad.
Also, as stated above, the contaminants often reside in the grooves and pores of the CMP pad, so in order to get samples of solvent that contain a representative amount of contaminants, the sample should be taken from within the grooves or pores. However, the pipette tips are generally too large to fit down into the narrow grooves on the CMP pad. As a result, the sample taken may be inaccurate, and the user will likely make an inappropriate assessment of the CMP pad cleanliness. Another problem is that the solvent may absorb into the CMP pad after it is poured. As a result, it may be difficult to collect the required amount of solvent for testing.
Hence from the foregoing, it will be appreciated that there is a need for a device and method that will simply and accurately measure the amounts of contaminants deposited on a CMP pad in order to determine its level of cleanliness.
The present invention satisfies the aforementioned needs. The disclosed method and apparatus permit easier, more accurate, and more controlled sampling of the contaminants on a CMP pad.
One aspect of the present invention comprises a method of determining the cleanliness of a CMP pad. First, the user selects a solution that will dissolve contaminants on the CMP pad. Then, the user positions a retaining structure on a first surface of the CMP pad so as to define a controlled space and injects a controlled volume of solution onto the CMP within the controlled space. Next, the solution with dissolved contaminants is extracted from the controlled space adjacent the CMP pad. Finally, the extracted solution is sampled to determine the quantity of dissolved contaminants in the CMP pad, and it is determined, based on this sampling of the extracted solution, whether the CMP pad meets a pre-selected cleanliness threshold.
Another aspect of the present invention is an assembly for obtaining a sample indicative of the cleanliness of a CMP pad having a first surface. The assembly comprises a retaining structure that is positionable adjacent the first surface so as to define an enclosed volume adjacent the first surface of the CMP pad. The assembly also comprises a reservoir containing a sampling solution and a delivery mechanism for delivering the sampling solution to the enclosed volume such that the sampling solution absorbs contaminants on the first surface of the CMP pad. Furthermore, the assembly comprises an extraction mechanism that extracts the sampling solution from the enclosed volume such that the extracted sampling solution contains contaminants of the first surface of the CMP pad. The sampling solution can then be evaluated to determine the cleanliness of the first surface of the CMP pad.
Most of the solution will remain inside the enclosed space in the retaining structure after it is injected therein. Advantageously, the solution is easy to collect for subsequent sampling. Also, the enlarged area of the enclosed space allows sampling of contaminants that may be present in multiple grooves, where there may be different levels of contaminants. As a result, the present invention advantageously allows the user to take a more representative sample of the level of contaminants present on the CMP pad.
Another aspect of the present invention comprises a retaining structure that is adapted to be positioned adjacent a surface of the CMP pad, and the retaining structure defines an enclosed volume that retains fluid within the enclosed volume adjacent the surface of the pad. In this embodiment, a pipette is used to inject cleaning solution onto the surface of the CMP pad within the enclosed volume. The pipette is also used to extract cleaning solution from the enclosed volume for subsequent analysis. The analysis determines the level of contaminants to thereby ascertain whether the CMP pad is suitable for continued use. Advantageously, use of this embodiment of the chemical collection assembly allows for easy set up and break down because the components involved are preferably light and portable.
A different aspect of the present invention comprises a delivery system for delivering cleaning solution to the surface of the CMP pad. This embodiment also comprises a retaining structure that is positionable with respect to the surface of the CMP pad so as to define an area on the surface of the CMP pad into which the delivery system positions the cleaning solution. Additionally, a recovery system that recovers cleaning solution from the area on the CMP pad is used. A controller sends signals to the delivery system and the recovery system so as to induce delivery and recovery of the cleaning solution.
Advantageously, use of this embodiment of the present invention allows for easy assessment of the CMP pad cleanliness because it is substantially automated. Also, the correlation between pad use and pad cleanliness can be discerned more quickly and with more confidence because human error is unlikely to affect the data collected from the cleanliness testing.
These and other objects and advantages of the present invention will become more apparent from the following description taken in conjunction with the accompanying drawings.